The production of PWBs includes a variety of techniques to deposit solder paste on a substrate. One method of depositing solder paste includes stenciling. This method includes the use of a stencil with cutouts in the stencil corresponding to the desired solder pattern for a PWB. The stencil, typically constructed of metal, is applied to the surface of a PWB and solder paste is applied to the stencil. A straight, rigid edge element, commonly referred to as a squeegee, is pressed down on the stencil and is wiped or moved across the stencil to deposit an even, smooth portion of solder paste into the cutouts of the stencil. Once the squeegee and stencil are removed, a solder pattern is left behind on the PWB.
The print quality of the PWB depends on the consistency of the dimensions and thickness of the solder paste after deposition. The dimensions and the patterns of the stencil in the stenciling process typically control the amount and thickness of the deposited solder paste. However, accurate deposition of the solder paste requires the stencil to be flush or in contact with the surface of the PWB as solder paste is deposited. Typically, the stencil is forced into contact with the PWB by the squeegee during the stenciling process.
Unfortunately, the non-coplanarity of PWBs significantly affects the print quality of the stenciling process because contact between the stencil and the PWB cannot be maintained during the stenciling process. As shown in FIG. 1, despite downward pressure from the squeegee, the stencil may not remain in contact with the PWB during stenciling if the warpage or non-coplanarity creates valleys or low lying depressions. As a result, printing quality may be insufficient to meet minimum standards, which results in additional production costs, repetition of work, and increased use and wear on production equipment.
The effect on print quality is particularly problematic for large PWBs. As the size of the PWB grows, the warpage or non-coplanarity typically worsens. This makes stenciling on large sized PWBs difficult with conventional equipment. Unfortunately, most large sized boards contain warpage and non-coplanarity characteristics that are incompatible with the use of conventional equipment, even if the boards meet standard specifications (0.75% max warpage per inch), such as IPC-2221 for surface mount technology.
The conventional equipment available for stenciling solder paste or adhesives on large size PWBs, such as boards greater than 18×24 inches, includes a metal stencil and a long, straight, rigid squeegee. The conventional squeegee is typically greater than 18 inches long and constructed from metal, generally stainless steel.
Stenciling large sized PWBs using a conventional long, straight squeegee results in unacceptable print quality because the squeegee is incapable of conforming to the non-coplanarity of the large sized boards. For example, as solder paste is spread over the stencil, a long, straight, rigid squeegee rides on the peaks of a warped large sized board without adequately pressing into the low lying areas of the board. As a result, conventional squeegees inadequately maintain contact between the stencil and the surface of the PWB. Consequently, deposition of solder paste onto low lying areas of a warped large sized board is inconsistent and insufficient to meet minimum print quality.
In previous attempts to overcome non-coplanarity have included increasing the downward pressure from the squeegee, using a flexible squeegee, and even trying to improve the coplanarity requirements on PWBs. Unfortunately, increased pressure from the squeegee results in damage to the stencil and the underlying PWB during the stenciling process. Further, increased pressure and friction between the stencil and squeegee results in significantly increased wear of and increased replacement of stenciling equipment.
Another attempt includes the use of flexible squeegees, which provide some ability to conform to the contours of the PWB. However, flexible squeegees are significantly less durable and more difficult to clean. While rigid squeegees provide a durable and consistent edge, which is necessary for uniform and accurate solder deposition, flexible squeegees have edges that degrade quickly under repeated use and cleaning.
Finally, attempts to require more consistent and coplanar PWBs are not practical for large sized PWBs. The increased cost of producing PWB with greater copalanarity is prohibitive, especially due to the fact that much of the warpage of the PWB is due to local heating and cooling during subsequent processing of the PWB.
Therefore, there exists a need for a squeegee capable of improving PWB print quality and compensating for the non-coplanar characteristics of large sized boards.